In today's clinical MRI, intensity of the main magnetic field ranges from 1 to 3 T, the number of voxels in one direction ranges from 128 to 512, and the image resolution is about 1-5 mm. The SNR is not a constant in different test samples even if the experimental setups are the same. A referred SNR in a 7 T MRI apparatus ranges from 20 to 40.
Nowadays, MRI is primarily used to display high-quality diagnostic images of human organs. Typical MR signals of clinic MRI fall in the radio-frequency range, and no ionizing radiation and associated hazards are expected. The spatial resolution of MRI is determined by the magnitude of the three gradient fields in three perpendicular directions. In general, the MR signals depend on the intrinsic parameters of the sample, including magnetization density M, spin-lattice relaxation time T1, spin-spin relaxation time T2, molecular diffusion and perfusion, susceptibility effects, chemical shift differences, and so on. The effects of these parameters on images can be suppressed or enhanced by adjusting certain operating parameters, such as repetition time TR, echo time TE, and flip angle. An MRI can display the spatial distribution of stationary magnetization density, relaxation times, fluid diffusion coefficients, and so on.
Prior to 1965, NMR spectrum was measured by observing the resonant absorption of RF radiation, either at fixed frequency while varying the main magnetic field (field-swept NMR), or at fixed main magnetic field while varying the frequency of excitation field (frequency-swept NMR). In 1965, Richard Ernst and Weston Anderson proposed an approach to measure the NMR spectrum by taking the Fourier transform on the measured free induction decay (FID) signal. In 1973, P. C. Lauterbur proposed the first MRI which is also Fourier based. Since the advent of Fourier-based NMR in 1965 and MRI in 1973, only Fourier-based techniques were proposed, possibly due to inheritance.
The prior MRI technology also well-known as Fourier domain MRI technology or frequency domain MRI technology is to detect the amplitude of the signal which is irrelevant to time. The Fourier MRI needs three gradient fields in three perpendicular directions. When the three gradient fields are set up, a specific voxel in a sample will resonate at a specific frequency. In addition, an ac excitation field is used to nutate the voxels which resonate at the same frequency. After the excitation field is turned off, the magnetization in the nutated voxel begins to relax and causes magnetic flux change which induces an FID signal in the detecting coil. The FID signal associated with the specific voxel is used for measurement or imaging.
Although the prior MRI technology is widely used in many fields, there are still many outstanding problems as follow. (1) The scan time for a slice composed of 512×300 voxels using the prior MRI is too long to acquire a precise imaging (about 2-3 minutes). (2) The circuitry of the prior MRI is too complicated due to repeating switching on/off of the three gradient fields to get a proper resonant frequency. Besides, the long scan time causes the precise imaging of moving animals and living organs (lungs, etc.) impossible. Hence, a transient imaging on a microsecond level is required to expand the application of MRI.
Therefore, it brings no delay to invent a method and a control device to circumvent all the above issues. In order to fulfill this need, the inventors have made an invent “METHOD OF TIME-DOMAIN MAGNETIC RESONANCE IMAGING AND DEVICE THEREOF.” The summary of the present invention is described as follows.